This invention pertains generally to the field of fluid control devices and particularly to the formation and use of microfluidic systems.
The manipulation of fluids in small volumes is required or desirable in many applications of microfluidic devices, including rapid bioassays, microchemical reactions, and chemical and biological sensing. For a review of such applications, see M. Freemantle, xe2x80x9cDownsizing Chemistry,xe2x80x9d Chem. and Eng. News, Vol. 77, No. 8, 1999, pp. 27-36. A microfluidic handling system utilizing microchannels is described in U.S. Pat. No. 6,193,647 to Beebe, et al. A variety of techniques have been used to pump, transport, position, and mix small liquid samples. Examples of such techniques include electro-osmotic flow, electrowetting, electrochemistry, and thermocapillary pumping. Surface properties, particularly surface wetting properties, have a significant effect on liquid behavior when very small volumes of liquid are manipulated. Such surface effect or capillary force is the basis of capillary pumping. Studies of structured surfaces consisting of either hydrophilic and hydrophobic stripes or patterned positive and negative surface charges show phenomena which can be exploited to control liquid motions in microfluidic devices. See, H. Gau, et al., xe2x80x9cLiquid Morphologies on Structured Surfaces: From Microchannels to Microchips,xe2x80x9d Science, Vol. 283, 1999, pp. 46-49; A. A. Darhuber, et al., xe2x80x9cMorphology of Liquid Microstructures on Chemically Patterned Surfaces,xe2x80x9d J. Appl. Phys., Vol. 87, 2000, pp. 7768-7775; A. D. Strook, et al., xe2x80x9cPatterning Electro-Osmotic Flow with Patterned Surface Charge,xe2x80x9d Phys. Rev. Lett., Vol. 84, 2000, pp. 3314-3317.
In accordance with the invention, flow of liquids is carried out on a microscale utilizing surface effects to control and direct the liquid. Liquid flow is guided by surface flow paths to maintain full laminar flow. The liquid stream can be supported and guided entirely by a flow path formed on facing surfaces such that the flowing stream is in contact only with the flow paths on the surfaces. Because the flowing stream is not in contact with other confining structure, such as sidewalls or the interior walls of channels or tubes, resistance to flow is minimized and laminar flow conditions may be maintained at higher flow rates. The surface tension of the flowing liquid provides vertical support for the stream of liquid, resulting in maximum exposure of the flowing stream to the atmosphere that surrounds it, thereby maximizing the surface area of gas-liquid interactions. Such microfluidically controlled streams may be utilized for applications such as chemical analysis, drug research, chromatography, cooling of electronic chips, flow sensors, air borne sample collection, and various medical applications including implantable drug dispensing systems and dialysis systems.
A microfluidic flow guiding structure for carrying out the invention includes a base having a surface and a cover with a surface facing the base surface. Adjacent facing regions on the base surface and cover surface define a flow path from a source position to a destination position on the base surface and cover surface, with at least a region on each of the base surface and cover surface being wettable by and having a wetting angle of less than 90xc2x0 with respect to a selected liquid, the wettable region on at least one of the base surface and cover surface formed as a flow guiding strip and a region adjacent to the guiding stripe on the at least one of the base surface and cover surface being non-wettable by and having a wetting angle of greater than 90xc2x0 with respect to the selected liquid. At such dimensions, a liquid injected onto the guiding stripe will be held by surface energy forces between the guiding stripe(s) on the base and cover surfaces and will flow along the stripe(s) from the source position to the destination position without flowing onto the non-wettable regions adjacent to the stripes. Further, the flow will remain laminar along the guiding stripes up to relatively high flow rates. The surface tension of the liquid itself creates a virtual wall that separates the liquid from the surrounding gas. Because of the smooth laminar flow of the liquid, there is no turbulence at the interface between the flowing liquid and the surrounding gas, and thus no intermixing of the liquid and gas occurs. The smooth interface between the gas and liquid phases along the flowing stream allows gas-liquid reactions to take place as a function of diffusion across the interface. Because the flowing stream is supported only at its bottom and top, the surface area of the flowing stream that is exposed to the ambient gas is maximized, and is much larger than the surface area per unit flow rate that can be obtained with liquid flowing through channels in contact with sidewalls or through channels formed in permeable membranes. Thus, chemical reactions between the gas and liquid can occur much more rapidly per unit volume by utilizing the present invention.
Two or more parallel guiding stripes that are separated by a non-wettable region may be formed on the base and cover surfaces to guide adjacent streams of liquid that flow together and that contact each other without mixing, but with diffusion allowed across the boundary between the two liquids.
The flow guiding stripes may be formed by various techniques, including lithography and the deposition of self-assembled monolayers by appropriately controlling flowing streams of liquid material, such as trichlorosilanes, which will form a non-wettable layer on the surface of the base. The flowing streams of material that deposit the monolayers may be guided by channels having a bottom wall and vertical walls that are formed in the base and closed by the cover surface, with the bottom wall of the channels defining the surface of the base on which the flow guiding stripes are formed.
Valves which control the flow of liquid on the flow guiding stripes may be formed in various manners, including as a barrier on the flow guiding stripe which blocks liquid flow below a selected pressure level; above that pressure level liquid on the guiding stripes will flow around the barrier. The valves may also be formed of materials that change dimension in response to characteristics of the liquid or that change over time from hydrophobic to hydrophilic or vice versa. Such materials include hydrogels that swell in response to certain conditions in the liquid to selectively block the flow on the guiding stripes.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.